Couples who get shut-eye together are better sleepers than those with different bedtimes, a study has found.
When your partner comes to bed after you, the disruption means it can take almost 40 minutes longer to fall asleep – and women are more affected than men by their other half having a different bedtime.
Researchers analysed 859 married couples' sleep over eight years and found rest is most affected during the 'fragile sleep window' and not by a partner getting into bed when already deep in slumber.
But, interestingly, people who go to bed later than their other half seem to have poorer quality of sleep, too.
The researchers suggest couples who go to bed together may feel emotionally closer, and going to bed at different times can make two people feel less connected, which may lead to stress and worse sleep.
The study, led by Seoul National University and published in BJPsych Open, which is the journal of the Royal College of Psychiatrists, looked at couples over the age of 60.
It found couples who shared a bedtime took no more than 24 minutes to fall asleep on average. But people whose partner came to bed while they were drifting off ended up taking more than twice as long to fall asleep - up to 63 minutes on average.
Researchers asked all couples for their typical bedtime and how long it took them to fall asleep - so they could work out if their partner's bedtime fell within the time they were still drifting off.
A study found couples who dozed off together were better sleepers than those with different bedtimes
Professor Ki Woong Kim, senior author of the study from Seoul National University, said: 'The problem with different bedtimes comes when one partner climbs into bed while the other is still trying to fall asleep - the fragile 20 to 30 minute window before sleep is established. During that window the brain is highly reactive to light, sound and movement, and a partner getting into bed effectively resets the clock on sleep onset.
'We always talk about sleep issues as an individual problem but most of us don't sleep as individuals - we share a bed with someone else. This means we need to look far more closely at mismatched bedtimes and how they affect sleep.
'Even for couples where one person goes to bed after the other is already fast asleep, having different bedtimes may make them less satisfied in their relationships, which may cause stress and sleeplessness.'
People whose partner came to bed when they were drifting off also spent more of the night lying awake in bed - approximately 14 per cent of the night on average. Meanwhile couples who shared a bedtime spent less than six per cent of the night awake.
People whose partner came to bed when they were already fast asleep were not significantly affected in terms of sleep, the study found. And those night owls who headed to bed to join their sleeping partner only took a little longer to fall asleep than people who shared a bedtime.
But these night owls had poorer sleep quality, which was judged using a questionnaire asking things like how frequently they wake up during the night and whether they struggled to stay awake at key points during the day.
The long-term effects of couples having different bedtimes are mostly felt by women, the study found. Tracking the Korean couples over eight years, the researchers found women whose partners came to bed later, while they were falling asleep, took about six minutes longer to fall asleep each year compared to the year before.
Professor Kim said: 'When many people, especially older people, are concerned about their sleep, going to bed at the same time as your partner is an easy tactic to try, which can be implemented straight away without sleeping pills.
'It is just a small change which could make a significant difference.'